The present invention relates to envelope detection circuitry, more particularly to envelope detection circuitry for use in low power communication devices employing amplitude modulation scheme.
Low power, high fidelity amplitude modulated receivers are used in numerous systems, such as portable AM radio receivers, two way radios, and remote control devices, telemetry radios and the like. Such portable devices typically operate on very little power in order to extend their usage, and accordingly require minimal power consumption during both signal demodulation and standby mode operation when no input signal is detected.
Radio frequency identification (RFID) represents another system in which amplitude modulated signals are used to remotely communicate information. As known in the art, RFID systems can be used in asset tracking and inventory management systems in which RFID tags (i.e., small RFID transceivers) may be place on an asset, and the asset's location monitored. RFID systems typically employ an On-Off Keying (OOK)/Amplitude-Shift Keying (ASK) based modulation scheme for down-link communications (from an RFID Reader/central station to the RFID tag) due to the simplicity in implementing the required receiver hardware. This modulation scheme alters the Reader output field strength as a way to convey information to the RFID tag. At the time when the radio field is reduced or completely cut-off, the RFID tag has to survive a momentary dip in field strength by preferably relying on stored charge in a capacitor to provide the needed DC power supply. While this process is advantageous in complementing battery-supplied power in “active” RFID tags, it becomes particularly important for “passive” RFID tags which do not include an on-board battery. Power is obtained from the radio field typically by rectifying the received radio wave into a supply voltage used to power the RFID tag and storing charge within a capacitor for powering the RFID tag over a short duration when the radio field has been reduced or cut-off due to downlink communication bit streams.
By sharing the same radio field with the communication channel, the ASK/OOK modulation scheme results in ripples on the power rectified output. Ripples on the rectified supply VDD can be detrimental to the functioning of the RFID tag circuitry. VDD ripples can be reduced by (i) increasing the capacitor size, (ii) decreasing the modulation index, (iii) increasing the data rate, or (iv) increasing the duty ratio.
Increasing the capacitor size is effective in smoothing out VDD ripple. However, a large capacitor would take up a significant amount of space, and if fabricated on an integrated circuit, would consume a significant amount of area and decrease IC yield. Decreasing the modulation index requires complex demodulation circuitry, and would be sensitive to environmental perturbations. Increasing the data rate or duty ratio has the same effect of reducing the time gap where the radio field is reduced or cut-off. While VDD ripples can be minimized by reducing the time gap for power dip, it also requires a high speed detector to trace the envelope of the fast changing signal. Power consumption of such envelope detector becomes a major concern as data rate or duty ratio increases.
FIG. 1 illustrates one approach for demodulating an OOK/ASK modulated signal from a received RFID radio field consisting of an envelope detector employing a rectifier circuit 110, with the same/similar architectures being employed in other amplitude modulated, low power receivers. A received modulated signal 105 is supplied to the signal rectifier 110, which produces the demodulated baseband signal 115. Capacitor C1 functions as a short circuit at radio frequency (RF) to filter out the RF carrier and its value is primarily determined by the carrier frequency. Depending upon the implementation, R1 can be replaced with a current sink transistor or multiple current sinks to drain the charge at Vx. The demodulated baseband signal Vx 115 is further converted into binary level data2bb by a low frequency comparator 120 or equivalent functional block. When Vx drops by a minimum falling threshold and reaches the reference value-ref, the comparator 120 output a binary 0. On the other hand, when Vx rises above the reference value-ref, the comparator output a binary 1.
FIG. 2 illustrates the signal waveforms at various nodes of envelope detection 100, with features of the FIG. 1 being retained. Shown are the envelope of the modulated signal 105 at the input, the rectified signal Vx 115, the demodulated baseband signal data2bb 202, and the power supply signal VDD 204. The rectified signal Vx 115 tracks the rise of the RF envelope 105, with falling characteristic set by R1 and C1. The asymmetrical rise time and fall time of Vx 115 results in reduction 206 of low period of the demodulated baseband signal data2bb 202, as shown. Fidelity of the demodulated baseband signal 202 can be improved by sharpening the falling characteristic of Vx where Vx drops from its peak value to the reference value-ref. This can be achieved by reducing R1 or increasing the current drain at Vx. This, in turn increases the loading of the RF port through the signal rectifier 110.
In view of these applications in which amplitude modulated, power-limited receivers are used, what is needed is an envelope detection circuit having improved demodulated signal fidelity and minimal power consumption requirements.